I think e I don't think non vengono mai usati da soli, ma vengono sempre seguiti da so, come nell'esempio, o da una proposizione come nel caso di I think/I don't think he went to the concert.
Many businesses, especially bars, display the sign munirsi dello scontrino alla cassa (obtain a receipt from the cash desk). This means you must first pay for what you intend to order to eat or drink, get a receipt for it, and then show it when you give your order.